GLOSSARY

Microvilli are tiny cell-membrane extensions increasing surface area, distinct from multicellular intestinal villi.

The small intestine is central to uptake of many nutrients and digestion by pancreatic and brush-border enzymes. The duodenum receives bile and pancreatic secretions; later regions also absorb extensively. Terminal regions specialize in processes such as bile-salt recovery and intrinsic-factor-dependent uptake. Regional distinctions do not mean every nutrient belongs exclusively to one tiny site or that all absorption finishes immediately after the stomach.

Villi are multicellular mucosal projections; microvilli are tiny extensions of individual absorptive cells. Microvilli form the brush border containing enzymes and transporters. Crypts are invaginations with renewing and specialized cells. These organizational levels differ. Greater surface supports absorption, but the appearance or name of a structure does not calculate the absorbed percentage of a person's meal or prove a supplement increases it.

For example, record amount, food form and time before training when assessing a meal; neither a label value nor one blood peak alone establishes complete absorption and use.

Brush-border enzymes on the lumen-facing intestinal surface complete particular digestive steps; not all digestive enzymes float freely in juice. Small products then enter through suitable routes. Reduced enzyme activity can alter tolerance of a specific substrate, but symptoms overlap. Discomfort after a meal does not identify the faulty enzyme or transporter by itself, and more enzyme activity is not automatically needed in healthy digestion.